This invention relates to getter positioning in a color cathode ray tube (CCRT) having an internal magnetic shield, and more particularly relates to a getter and contact assembly and associated mounting means for attachment to the shield of such tube.
While the invention is useful in conventional CCRT's, it is particularly useful in high resolution CCRT's for displays having an automatic convergence system. Such displays have application in such demanding fields as computer aided design (CAD) and cartography. See ELECTRONIC PRODUCTS, May 12, 1983, p. 17. Essential to such an autoconvergence system are certain feedback features in the CCRT, which provide information on the location of the scanning electron beams to external correction circuitry, which then correct any misconvergence of the beams. Such feedback features include a phosphor pattern on the rear or gun side of the tube's aperture mask, and a window in the side of the tube. When impinged by the scanning electron beams, the rear-oriented phosphor pattern emits radiation, a portion of which is transmitted through the window and detected by an externally positioned photomultiplier tube.
The window must not only be transparent to the emitted radiation but also must be sufficiently conductive to prevent localized charge build-up, which could distort the adjacent potential field, resulting in disturbance of the trajectories of the electron beams. A suitable window structure is described and claimed in co-pending U.S. patent application Ser. No. 448,468, filed Dec. 10, 1982, assigned to the present assignee.
The getter flash, which is an internal deposit of gas-adsorbing material essential to adequate life of the CCRT, must be distributed in the feedback CCRT in a manner to avoid both the phosphor pattern on the back of the mask and the window, to assure an adequate signal to the photomultiplier tube. Such a distribution is achieved in a getter structure described and claimed in U.S. patent application Ser. No. 449,897, filed Dec. 15, 1982, assigned to the present assignee.
The feedback CCRT shares a common problem with other CCRT's, that is, susceptibility to high surge currents caused by internal arcing. Such susceptibility to arcing is not surprising in view of typical operating potentials as large as 25 to 30 kilovolts, and the large potential differences between various tube components, especially the closely spaced gun electrodes. Steps are taken during tube manufacture to minimize arcing during subsequent tube operation, especially high voltage conditioning in which a voltage of 40 kilovolts or more is applied between the terminal high voltage electrode and the adjacent electrode of the electron gun to remove projections and foreign matter from the inter-electrode spacing. Despite this and other precautions, occasional arcing does occur, resulting in momentary surge currents as high as 400 amps, which can be devastating to electrical components in the associated circuitry outside the CCRT. Thus, numerous structures have been proposed to reduce or dissipate surge currents inside the CCRT. These involve internal high resistance coatings in the neck and funnel regions of the tube, the positioning of resistive means between the internal conductive coating and the convergence cup, the placing of resistors between various gun components, and the discrete positioning of getters to avoid shorting of the internal coating by the getter or getter flash.
The effectiveness of high resistance coatings in the neck region may be reduced or eliminated by the getter assembly or getter flash or both forming a conductive bridge across the coating. Solutions offered to avoid this problem include moving the getter away from the neck region, for example, to the mask (U.S. Pat. No. 3,979,633). However, moving the getter to the mask results in getter flash deposits on the back side of the mask. This is, of course, undesirable in the feedback CCRT.
A solution to the problem is presented in U.S. patent application Ser. No. 525,758, filed Aug. 23, 1983, assigned to the present assignee, wherein a CCRT incorporates an arc suppression coating in the neck region of the tube, and a getter structure affixed to the internal magnetic shield (IMS). The getter structure is constructed and positioned to achieve a getter flash distribution that substantially avoids the mask and neck regions, as well as the window region of the feedback CCRT.
The getter structure has been affixed to the shield prior to sealing of the panel and funnel portions of the CCRT envelope, in the conventional manner. However, this procedure results in the getter being subjected to the elevated temperatures required for sealing, and getters which are known to withstand such sealing temperatures are not among the most efficient available. Therefore, to attain high quality tube performance and long tube life, the getter is desirably introduced to the tube subsequent to sealing. Techniques for achieving this are known in the art, by using an insertion tool to insert the getter through the open neck of the tube, and to mount the getter on the mask frame or anode button. Frame-mounted getters are not suitable for feedback CCRT's for the reason already mentioned, i.e., getter flash tends to deposit on the back side of the mask. Button-mounted getters require a specially designed anode button, and would not necessarily result in the required distribution of getter deposit for the feedback CCRT.